The present invention relates generally to apparatus and a method for processing video signals comprising chrominance and luminance signals and, more particularly, to video signals as generally processed in a video tape recorder wherein a separate channel is used for recording a chrominance signal on a magnetic tape.
As is known, the spectrum of a luminance (and sync) television signal includes a dc component and components at harmonics of the horizontal scan frequency, each with a cluster about it of components separated at the vertical scan rate. Chrominance information is generally transmitted in the spectral gaps between these components. In the NTSC system, for example, two components of the chrominance signal, the in-phase (I) and quadrature (Q) signals, are amplitude modulated in quadrature to each other onto a subcarrier whose frequency (3.579545 MHz) has been selected to cause the resulting chrominance sidebands to fall in the spectral gaps between the luminance spectral components. The composite signal thus includes interleaved luminance and chrominance signals. The subcarrier frequency results in horizontal, vertical, and temporal interleaving so as to minimize mutual interference from cross-talk between the luminance and chrominance signals.
In typical video recording systems such as video cassette recorders (VCR's) used in the home for recording television signals, it is known, for the purpose of recording on a magnetic tape, to transpose the chrominance signal conventionally situated in the upper portion of the frequency spectral band of a composite television signal to a position in the spectral band below the luminance signal. Such a modulation or down conversion is commonly known in the recording art as a "color-under" recording system. In such a color-under system, the in-phase (I) and quadrature (Q) components of the chrominance signal are processed conventionally, albeit, at a different carrier frequency to develop a color signal (C) for displaying a scene.
The well-known VHS system utilized in many VCR's produces degraded picture quality in comparison with, for example, properly received and processed broadcast television signals because it does not provide the full necessary horizontal resolution. It has long been a goal of television engineers to find ways of transmitting more information in a channel of given bandwidth. The frame and line rates are generally fixed in accordance with standards, and therefore, a bandwidth limitation typically results in a reduction in horizontal resolution. The typical restricted bandwidth of, for example, the VHS system of about 2.0 to 2.5 MHz thus produces an image of inadequate horizontal resolution.
Various techniques are known for recording a full bandwidth signal on a limited bandwidth medium such as magnetic tape. U.S. Pat. No. 5,113,262 issued 12 May 1992 to C. H. Strolle et alii, entitled VIDEO DIGNAL RECORDING SYSTEM ENABLING LIMITED BANDWIDTH RECORDING AND PLAYBACK, and herein incorporated by reference, discloses such as system. Furthermore, a number of prior U.S. patent applications and other publications are discussed in U.S. Pat. No. 5,113,262 of Strolle et al. Related subject matter is also disclosed in the following patent applications, herein incorporated by reference: U.S. patent application Ser. No. 07/562,907, filed Aug. 6, 1990, of Ko et al., entitled REMOVAL OF THE FOLDING CARRIER AND SIDEBANDS FROM AN UNFOLDED VIDEO SIGNAL; and U.S. patent application Ser. No. 07/531,070, filed May 31, 1990 of Strolle et al., entitled COLOR-UNDER CHROMA CHANNEL ENCODED WITH AUXILARY SIGNALS. U.S. Pat. No. 5,083,203 issued Jan. 21, 1992, to J. W. Ko and Hermann J. Weckenbrock, entitled CONTROL SIGNAL SPREADER, and incorporated herein by reference also discloses subject matter relevant to the U.S. Pat. No. 5,113,262. Other systems, such as, for example, that disclosed in U.S. Pat. No. 4,831,463, issued May 16, 1989 to Faroudja, herein incorporated by reference, require modification of the format such that a tape recorded in accordance with that system would result in undesirable artifacts or defects in the picture when played back on a standard unmodified VHS machine.
In a system such as that described in the afore-mentioned pending U.S. patent application Ser. No. 07/569,029, filed Aug. 17, 1990 of Strolle et al., entitled AN IMPROVED VIDEO SIGNAL RECORDING SYSTEM, (Samsung Docket No. 1020) backward compatibility is available for video signals recorded in the improved (I-VHS) format and such recorded signals can be played back on a standard VHS player. In setting up system parameters, a compromise can be struck between the backward compatibility which is often a desirable advantage and the picture quality which may be somewhat compromised by artifacts under certain conditions. Thus, the quality of the I-VHS picture is generally less than optimum possible without constraints because of the need to provide backward compatibility. Nevertheless, conditions may arise when one or the other of these two conflicting requirements is the more important. For example, a user of a camcorder may wish to decide that backward compatibility for playback on a standard VHS machine, while nevertheless desirable and required, is not a factor of such major importance in a particular application that it should be allowed to compromise unduly a picture intended to be reproduced from the same recording medium on an I-VHS machine. Conversely, a user may wish to decide that a high degree of backward compatibility is so desirable in another particular application that the quality of reproduction when using an I-VHS machine need not be the utmost. However, in both instances, the user is constrained to accept the built-in engineering compromise the manufacturer has opted to design into the equipment as offering a generally acceptable solution under various conditions of use.